1. Field of the Invention
The present invention relates to an optical scanning apparatus used in an image forming apparatus such as a laser printer, a digital copying machine, or a multifunction printer.
2. Related Background Art
Ordinarily, optical scanning apparatuses for use in these kinds of image forming apparatus operate in such a manner that a beam of light from a laser light source is deflected by a polygon mirror and travels through an imaging lens system to thereby form an imaging light spot on a surface to be scanned.
There are many cases where a semiconductor laser or the like is used as a laser light source in a manner described below. Divergent rays of light emitted from a laser light source are converted into an approximately parallel light beam by a collimator lens and the light beam is shaped by an aperture. The shaped light beam enters an imaging lens system after being deflected by a polygon mirror rotating at a constant angular velocity. It is required that the imaging lens system have an fθ characteristic to cause a scanning surface placed at a certain distance from the imaging lens system at a constant distance speed with the light beam deflected at a constant angular velocity by the polygon mirror. It is also required that the curvature of field be suitably corrected so that a small light spot can be formed at any point in the entire scanning area.
Ordinarily, the imaging lens system is arranged to have a tilt correction function for correcting a deviation of the scanning position in a direction perpendicular to a main scanning direction, i.e., in a sub-scanning direction, because the polygon mirror has an error in its mirror surfaces caused at the time of working for forming the surfaces, and because the rotating shaft of the polygon mirror vibrates. Therefore, the imaging lens system is formed as an anamorphic lens system having imaging characteristics differing between the main and sub-scanning directions.
Conventionally, the imaging lens system has lenses formed from a glass material so as to have a toric surface and a cylindrical surface. Such lenses have an antireflection coating formed thereon by vapor deposition or the like. In recent years, as such lenses, low-cost plastic lenses capable of being freely shaped to correct aberrations have ordinarily been used since working of glass lenses is difficult to perform and the working cost is high.
Semiconductor lasers conventionally used as light sources are infrared (780 nm) lasers and visible light (675 nm) lasers. Therefore, polygon mirrors or bending mirrors formed of copper mirrors having a high reflectance while having low wavelength dependence and low angle dependence have been used.
FIGS. 8A and 8B show reflectance characteristics of copper itself, and FIGS. 9A and 9B show reflectance characteristics of a conventional copper mirror which is formed in such a manner that a copper film is formed on an aluminum base member and alumina (Al2O3) and SiO2 are vapor-deposited on the copper film. As can be understood from these figures, the mirror has improved reflection characteristics in the infrared laser and visible light laser wavelength bands.
Further, in response to demands for image forming apparatuses of higher resolution, the development of optical scanning apparatuses capable of forming a smaller shaped spot is being advanced.
However, it is apparent from FIGS. 8A, 8B, 9A and 9B that, with decreasing wavelength, the reflectance of the copper mirror decreases and its wavelength and angle dependences are also increased. In use of the copper mirror with a short-wavelength laser in the conventional system, it is necessary to increase the laser power or to use a collimator lens of a smaller F number in order to maintain a predetermined quantity of light. In such a situation, the load on the laser itself is considerably heavy and there are cost-increasing factors, e.g., an increase in the number of collimator lenses for suitably correcting aberrations.
In use of a semiconductor laser as a light source in a certain operational environment, variation in wavelength is inevitable because of a temperature-dependent oscillation wavelength characteristic of the laser. It is, therefore, required that variations in the optical characteristics, i.e., the transmittance, the reflectance, etc., of optical components used in the scanning optical system be small in the vicinity of the laser oscillation wavelength. While the copper mirror has a good characteristic with respect to infrared laser light and visible laser light, considerable fluctuation in light quantity, i.e., image density nonuniformity, results from the wavelength-dependent reflectance of the copper mirror.
Moreover, because of the angle dependence of the reflectance, the uniformity of image density between the scanning center and scanning end is far from sufficient for formation of a high-quality image.
In general, optical materials used for forming plastic lenses have such a transmittance characteristic that, with decreasing wavelength, the transmittance decreases due to absorption in the material. High-cost glass lenses have, therefore, been used in optical scanning apparatuses using short-wavelength light sources.
FIG. 15 shows a graph showing the transmittance of ordinary optical resins. Variation in the transmittance due to internal absorption in the vicinity of the oscillation wavelength (780 nm) of an infrared laser or the oscillation wavelength (675 nm) of a visible light laser conventionally used as a light source is negligibly small. In the case of use with a light source of a short wavelength in the vicinity of 400 nm, however, the reduction in transmittance due to internal absorption is not negligible. Also, since the ray passage distance in the plastic lens changes with respect to the image size, deterioration in image quality due to light quantity distribution nonuniformity at the position on the scanned image surface is more considerable than the reduction in absolute quantity of light.
Also, in use of a semiconductor laser as a light source in a certain operational environment, variation in wavelength is inevitable because of a temperature-dependent oscillation wavelength characteristic of the laser. It is, therefore, required that variations in optical characteristics, i.e., the transmittance, the reflectance, etc., of optical components used in the scanning optical system be small in the vicinity of the laser oscillation wavelength. In a case where a plastic lens is used in a short wavelength range in the vicinity of 400 nm, there is a problem of image density nonuniformity which, as can be understood from FIG. 15, results from variation in the quantity of light on the scanned surface due to the wavelength dependence of the transmittance.